Our laboratory will continue its research on the effects of irradiation on mammalian embryonic development. These projects are as follows: 1. The Effect of Embryonic X-Irradiation on Leukemia Induction in Prenatally Exposed Mice (ongoing): These studies involve the exposure of strains of leukemia-sensitive and leukemia-resistant mice to prenatal radiation. The animals are studied for the postnatal occurrence of leukemia; 2. Determining the Threshold Dose or Low-Dose Incidence for Congenital Malformations in the Developing Rat Embryo (ongoing): Our studies involve much larger numbers of animals than have been utilized in previous investigations and detailed analysis of the internal and external morphology. The threshold studies proposed should provide important information on the frequency of malformations at low doses; 3. The Effect of X-Irradiating the Rat Embryo on the First Day of Gestation: Determination of those Factors or Circumstances which are Responsible for Embryonic Death; 4. The Effect of Neutron Radiation on Embryonic Development in the Rat; 5. The Effect of X-irradiation on Cell Kinetics of the Developing Rat Embryo during Organogenesis: Utilizing the techniques of the cell biologist, the cell cycle parameters will be studied in irradiated and control embryos during the period of organogenesis. Dissociated early embryos will be studied for cell number, cell volume and cell viability. Autoradiographic techniques with tritiated thymidine will be used to describe the cell kinetics; 6. The Indirect effect of Embryo-Culture as a Method of Evaluating the Mechanism of Radiation Teratogenesis; 8. Irradiation of the Developing Yolk Sac: Specific antibodies against various components of the developing yolk sac will be labeled with 211At or 125I in an effort to specifically irradiate the developing rat yolk sac. Because of the nature of radiation embryological effects, a great deal of information obtained in radiation embryological experiments can be prudently applied to the human situation.